Magnesium aluminate (MgAl.sub.2 O.sub.4) is a transparent, chemically stable material having a low permittivity and good resistance to electric and thermal shocks. Magnesium aluminate may be coated on a silicon substrate to form a single crystal film thereof as a buffer layer before coating the substrate with an epitaxial perovskite-type oxide film made of strong dielectrics such as PbTiO.sub.3, BaTiO.sub.3 and SrTiO.sub.3, or of a high temperature superconductor based on copper oxides. The magnesium aluminate buffer layer located between the substrate and the superconductor or dielectric layer may ameliorate many of the problems which occur when the substrate is directly coated with the superconductor or dielectric layer. For example, the magnesium aluminate layer may reduce stress caused by differences in the thermal expansion coefficient and crystal lattices between the coating layer and the substrate. Also, it may prevent the coating material from reacting with the substrate.
There have been reported a number of methods for the preparation of a magnesium aluminate film by using a magnesium aluminum alkoxide represented by MgAl.sub.2 (OR).sub.8 wherein the aluminum to magnesium atomic ratio is two as in magnesium aluminate (see U.S. Pat. Nos. 3,761,500 and 3,786,137). Further, the use of an isopropoxide derivative thereof, which is volatile at a low pressure, in chemical vapor deposition (CVD) of a magnesium aluminate has been recently reported (see Jiming Zhang et al., Journal of Materials Research, 9, 1333 (1994); and Richard E. Rocheleau et al., Chemistry of Materials, 6, 1615 (1994)).
However, it has been reported that in the preparation of the isopropoxide derivative mentioned above, higher molecular weight by-products, e.g., {Mg[Al(OPr.sup.i).sub.4 ].sub.2 }.sub.n and Mg.sub.2 Al.sub.3 (OPr.sup.i).sub.13 are also produced (see Julian A. Meese-Marktscheffel et al., Chemistry of Materials, 5, 755 (1993)). The formation of such high molecular weight by-products leads to lowering of the vapor pressure of MgAl.sub.2 (OPr.sup.i).sub.8 in a CVD process to deposit a magnesium aluminate film. Moreover, the above-mentioned magnesium aluminum alkoxide derivative has the disadvantage that it must be heated to about 200.degree. C. or higher when it is used in a CVD process.
A magnesium alkylaluminum alkoxide having the general formula of Mg[(OR).sub.2 AlR.sub.2 ].sub.2 was first synthesized in the form of a dioxane complex by J. L. Atwood et al. in 1968 (see Journal of Organometallic Chemistry, 13, 53(1968)). Recently, Chang et al. reported that [Me.sub.2 Al(.mu.--NPr.sup.i.sub.2).sub.2 MgMe].sub.4 and [Me.sub.2 Al(.mu.--NEt.sub.2).sub.2 MgMe].sub.4 may be reacted with t-butanol to produce a mixture of Mg[(.mu.--OBu.sup.t).sub.2 AlMe.sub.2 ].sub.2 and [Me.sub.2 Al(.mu.--OBu.sup.t).sub.2 Mg(.mu.--OBu.sup.t).sub.2 ].sub.2 (see Cheng-Cheng Chang et al., Journal of Chemical Society, Dalton Transactions, 315 (1994)). However, the above method has problems in that the yield is low, the purity of the intended product is low and the starting materials are not easily available.